As a change of pace from classic mashed potatoes, I like to serve a puree of celery root because its sweet, mild celery flavor complements roasted meats, turkey, or game. Celery root, also called celeriac or knob celery, is in season from November until April. Its delicate flavor more than makes up for its ugly looks! Look for large, hard, and relatively smooth-skinned knobs to minimize waste when peeling. A sharp knife, rather than a vegetable peeler is the best tool for removing the skin. There is a certain amount of waste because the skin is bumpy and grooved, which is why I prefer to buy larger knobs. Store celery root in the refrigerator until needed; it keeps for a week or so.
2 large (about 3 pounds total) celery roots
2 (about 1 1/4 pounds total) russet potatoes
1 teaspoon kosher or sea salt plus more for seasoning
4 ounces (1 stick) unsalted butter at room temperature, cut into chunks
1/2 to 3/4 cup heavy (whipping) cream warmed
Freshly ground white pepper
Fill a 6-quart saucepan two-thirds full of water and set aside. Using a sharp knife, cut off the top and a thin slice from the base of the root. Pare away the tough outer skin of the celery root and then cut it into 1-inch chunks. Immediately add it to the water in the pot to keep it from browning. Using a vegetable peeler, peel the potatoes, cut them into 1-inch chunks, and add them to the pot with the celery root. Add a teaspoon of salt. Partially cover the pot and bring the water to a boil over high heat. Reduce the heat so the water simmers and cook until the vegetables are tender when pierced with a knife, about 15 minutes.
Drain the vegetables in a colander and return them to the pot. Place the pot over low heat for 1 minute to evaporate any excess moisture.
In a food processor fitted with the metal blade, process half of the vegetables just until pureed and then transfer to a mixing bowl. Repeat to puree the second batch. Whisk in the butter, a chunk at a time, until incorporated. Gradually add the cream and whisk until the mixture is smooth. Add salt and pepper to taste. Serve immediately, or keep warm in the top of a double boiler, or cover and rewarm in a microwave oven.